Hadoop编写矩阵实例及演示

下面是一个使用Hadoop编写矩阵乘法的示例：

1. 首先，需要创建一个输入矩阵A和一个输入矩阵B。这些矩阵可以存储在HDFS中，每行一个元素，使用逗号分隔。

2. 然后，我们需要编写一个MapReduce程序来执行矩阵乘法。Map函数将输入矩阵A的行作为键，列和值作为值，并将其传递给Reduce函数。Reduce函数将输入矩阵B的列作为键，行和值作为值，并将其与Map函数传递的值相乘。

3. 最后，我们需要运行MapReduce作业并将结果存储在HDFS中。结果将是一个输出矩阵C，其中每个元素都是A和B的相应元素乘积的总和。

以下是示例代码：

public class MatrixMultiplication {

  public static class Map extends Mapper<LongWritable, Text, Text, Text> {
    private Text outputKey = new Text();
    private Text outputValue = new Text();

    public void map(LongWritable key, Text value, Context context)
        throws IOException, InterruptedException {
      String[] matrixRow = value.toString().split(",");
      String matrixName = matrixRow[0];
      String row = matrixRow[1];
      String col = matrixRow[2];
      String val = matrixRow[3];

      if (matrixName.equals("A")) {
        for (int i = 0; i < 10; i++) {
          outputKey.set(row + "," + i);
          outputValue.set(matrixName + "," + col + "," + val);
          context.write(outputKey, outputValue);
        }
      } else {
        for (int i = 0; i < 10; i++) {
          outputKey.set(i + "," + col);
          outputValue.set(matrixName + "," + row + "," + val);
          context.write(outputKey, outputValue);
        }
      }
    }
  }

  public static class Reduce extends Reducer<Text, Text, Text, Text> {
    public void reduce(Text key, Iterable<Text> values, Context context)
        throws IOException, InterruptedException {
      Map<String, Float> mapA = new HashMap<String, Float>();
      Map<String, Float> mapB = new HashMap<String, Float>();

      for (Text value : values) {
        String[] matrixRow = value.toString().split(",");
        String matrixName = matrixRow[0];
        String row = matrixRow[1];
        String col = matrixRow[2];
        String val = matrixRow[3];

        if (matrixName.equals("A")) {
          mapA.put(col, Float.parseFloat(val));
        } else {
          mapB.put(row, Float.parseFloat(val));
        }
      }

      float result = 0.0f;
      for (String col : mapA.keySet()) {
        if (mapB.containsKey(col)) {
          result += mapA.get(col) * mapB.get(col);
        }
      }

      String[] outputKeyArr = key.toString().split(",");
      String outputKey = outputKeyArr[0] + "," + outputKeyArr[1];
      context.write(new Text(outputKey), new Text(Float.toString(result)));
    }
  }

  public static void main(String[] args) throws Exception {
    Configuration conf = new Configuration();

    Job job = Job.getInstance(conf, "matrix multiplication");

    job.setJarByClass(MatrixMultiplication.class);
    job.setMapperClass(Map.class);
    job.setReducerClass(Reduce.class);

    job.setOutputKeyClass(Text.class);
    job.setOutputValueClass(Text.class);

    FileInputFormat.addInputPath(job, new Path(args[0]));
    FileOutputFormat.setOutputPath(job, new Path(args[1]));

    System.exit(job.waitForCompletion(true) ? 0 : 1);
  }
}

这个示例程序将输入矩阵A和矩阵B作为命令行参数传递，并将输出矩阵C存储在HDFS上的指定目录中。